The invention is directed to an optical receiver for receiving digitally transmitted data.
When digital signals with a high bit sequence frequency (gigabit range) are transmitted, various dispersion-effects occur caused by optical fibre cables in the optical transmission system. These dispersion effects distort the data signals, thus limiting the transmission lengths. Distortions can also occur on the optical fibre due to non-linear effects, e.g. the Kerr effect. Optical receivers used in such a transmission system are required to evaluate distorted signals, permitting only low bit error rates. It is known from the publication "Decision-point steering in optical fibre communication system", M. Sherif, IEEE Proceedings, Volume 136, No. 3, 1989, page 169 ff, that a receiver in a digital optical transmission system can be optimized by modifying decider thresholds, thus generating artificial errors. For this purpose a second decider circuit is applied parallel to the main data flow. This second signal branch is subjected to a large number of bit errors. The artificial error rate is achieved in the publication in question by modifying the decider threshold, which is changed by means of a square pulse. The results of the two deciders are compared, and from the result actuating variables are derived for an amplifier and for the receiving photodiodes. If you examine the eye diagram of the signal received, the first decider scans the eye aperture at an almost optimal point; i.e. both the decider threshold and the scanning window are roughly in the center of the open eye. The second signal path changes the decider threshold so that the threshold can already be in the noise of the eye diagram, and so artificial errors occur. By evaluating these artificial errors the whole optical receiver is readjusted to optimize the eye aperture, noise characteristics, etc.
A digital optical message transmission system is disclosed in EP 0554 736 B1 in which the receiver quality is optimized by evaluating the eye diagram. In this case an eye diagram is recorded in the receiver, and the size of the eye aperture is determined in a computer. The computer determines an optimum setting of the photodiode and the decider threshold from the data recorded. Recording an eye diagram, comparing with a stored nominal value, and adjusting various receiver activating variables does not enable the bit error rate to be directly determined and a required resultant optimization of the receiver.